This invention relates to the art of measuring intravascular blood pressure and, more particularly, to apparatus for directly measuring blood pressure at the location of interest by means of a catheter having a transducer tip insertable into the blood stream.
Catheters have been used in the art for monitoring variations in pressure within the blood stream at various locations throughout the cardiovascular system. It has been accepted practice to insert the distal end of a catheter into a blood vessel and then connect the proximal end of the catheter outside the body to an external transducer at which pressure variations are measured. Such external transducers are considered relatively inexpensive and employ disposable fluid filled catheters. However, in use such catheters require periodic flushing to avoid thrombus formation. Also, the frequency response characteristics are often compromised by the mechanical properties of the catheter, inclusion of small air bubbles and by body motion artifact. The blood pressure must be transmitted from the interior of the blood vessel through the catheter tubing by means of the fluid column therein before it can act on the external transducer. Consequently, the frequency response for such an external transducer is also compromised by the relatively large mass of the fluid column within the catheter tubing.
Other approaches to measuring intravascular blood pressure have included employing catheter tip transducers insertable into the blood stream. Such catheter tip transducers provide direct pressure monitoring in that they transduce blood pressure at the region of interest rather than attempting to couple the dynamic waveform hydraulically, as in the external transducers. Many of the catheter tip transducers employ semiconductors and other sensing elements of the resistive and/or capacitive variety at the catheter tip. An electrical signal is generated or modulated at the transducer and transmitted through the length of the catheter to meters and the like located externally of the body being tested. Such semiconductor tip transducers are expensive and, hence, the high cost is not compatible with their being disposable units. Instead, there is a tendency to reuse the product and, despite sterilizing or autoclaving, there remains a potential to transfer proteins, which may be antigenic, from one patient to a successive one. Another potentially troublesome feature of such semiconductor transducer tips is the use of electricity to power the sensor. The use of electricity not only renders the device susceptible to electromagnetic interference, but also introduces the possible hazard of arrhythmia induction.
To overcome some of the noted difficulties, other attempts in determining blood pressure in a cardiovascular system have included catheters employing optically based pressure transducer tips at the distal end. Such devices typically take the form as illustrated in U.S. Pat. No. 3,249,105 to Polyanyi, and Franke U.S. Pat. No. 3,215,135. Each of these devices employs a catheter having fiber optic means extending the length of the catheter to the distal end thereof at which the fiber optic means is in optical communication with a pressure transducer. The pressure transducers in Polyanyi and Faranke, supra, take the form of a diaphragm covering the end hole of the catheter. The diaphragm is located in front or distal to the end of the fiber optical means and then receives light and reflects it back into the fiber optic means for transmission to an externally located meter. Since the transducer tip is inserted into the blood stream of a patient, the blood pressure deflects the diaphragm causing modulation of the light intensity so that the meter provides an indication of blood pressure. Such catheters employing diaphragm covered end holes actually measure total pressure rather than the desired measurand; namely, static pressure. Thus, by aligning the end hole of a catheter with the direction of blood flow, kinetic energy terms are introduced. If the catheter end hole is directed upstream, the kinetic term will be added to the pressure, and, if the end hole is facing downstream, the kinetic term will be subtracted from the pressure. The magnitude of the error will vary with flow rate. This error will vary during the course of a cardiac cycle and will distort the shape and magnitude of a pressure wave. In the pulmonary artery, the kinetic pressure may be on the order of 10% of total pressure at rest and 50% of total pressure at a cardiac output equal to three times that at rest. The importance of the kinetic pressure error is particularly great in stenotic areas where velocities are high.
The noted problems with catheters employing diaphragm covered end holes to monitor pressure may be alleviated with a tip transducer that employes side port monitoring of pressure rather than end hole monitoring of pressure. The kinetic contribution is minimal when measuring pressure perpendicular to the blood flow. One such device is known and is reported in a 1978 article entitled "The Development of Fiber Optic Catheter Tip Pressure Transducer", Journal of Medical Engineering and Technology, Vol. 2, No. 5, by H. Matsumoto and M. Saegusa. As disclosed in that article, the Matsumoto optical sensor employs a tipped transducer having side port monitoring of pressure. The pressure transducer measures pressure acting at right angles to blood flow. A membrane is responsive to the pressure and causes movement of a mirror, which is mounted in cantilevered fashion to the membrane, within the cavity of the transducer tip. The mirror serves to reflect light received from fiber optic means extending the length of the catheter so that the intensity of light returned by way of the fiber optic means to a measuring device located outside the body is modulated in accordance with the static pressure. However, this structure requires precise alignment between the distal end of the fiber optic and the cantilevered mounted mirror so that the deflected light as measured by the externally located meter will be properly indicative of the static pressure. Additionally, there is a nonlinearity in the response characteristics of such a device both because the cantilevered mounted mirror undergoes an angular displacement and nonlinear displacement.